1. Field of the Invention
This invention relates generally to rod rolling mills, and is concerned in particular with an improvement in the laying heads used to form the rods exiting from such mills into helical formations of so called "rings".
2. Description of the Prior Art
Referring initially to FIGS. 1 and 2, the typical delivery end of a rod mill is depicted schematically as including the last roll stand of a finishing block 10, several water boxes 12, 14, a pinch roll unit 16, a laying head 18, a cooling conveyor 20 and a reforming tub 22. The finished rod exits from finishing block 10 at an elevated temperature in the range of 650.degree. to 950.degree. C. The rod is quenched in the water boxes 12, 14 before passing through the pinch roll unit 16 into the laying head 18. The laying head forms the product into a helical formation of rings 24 which are received on and transported along the length of the conveyor 20 towards the reforming tub 22. While on the conveyor, the rings are arranged in a Spencerian formation and subjected to various heat treatments, e.g., controlled cooling at selected rates to achieve selected metallurgical properties. The rings drop from the delivery end of the conveyor into the reforming tub 22 where they are gathered around a vertical mandrel 26 into upstanding cylindrical coils. Other devices (not shown) remove the coils from the tub for transport to other locations.
It is important to maintain a uniform ring pattern on the conveyor 20, with the diameters of the rings being such that they will drop smoothly into the reforming tub without being caught up on either the outer tub circumference or the central mandrel 26. Either of such occurrences will foul the equipment, necessitating a costly interruption in production.
While a segment of the rod is being rolled in the mill and another segment of the rod is passing through the laying head where it is being formed into rings that are being deposited on the conveyor, operating conditions remain substantially steady. Thus, a uniform ring pattern can be maintained by synchronizing the operating speed of the laying head with the speed at which the rod is being delivered from the mill.
When the tail end of the rod leaves the mill, the pinch roll unit 16 comes into play and serves to continue stabilizing operating conditions. Thus, when smaller diameter products on the order of 5.5 mm are being rolled at higher speeds in the range of 100 m/sec, the pinch roll unit will act as a brake resisting the tendency of the product to speed up after the tail end drops out of the finishing block. By the same token, when larger diameter products on the order of 12-20 mm are being rolled at slower speeds ranging from 11 to 30 m/sec, the pinch roll unit will serve to continue propelling the product through the laying head after the tail end drops out of the finishing block.
A problem arises, however, when the tail end of the product leaves the pinch roll unit 16. The distance "d" between the pinch roll unit and the delivery end of the laying head 18 is typically about 4 meters, which is equal to or slightly greater than the circumference of one ring being deposited on the conveyor. Under high speed operating conditions, when the tail end clears the pinch roll unit, that relatively short product length tends to speed up, causing buckling and/or an increase in the diameter of the last ring. This in turn can hinder passage of the last ring downwardly into the reforming tub.
A related although somewhat different problem can be experienced with the front end of the product as it leaves the laying head and before it contacts the conveyor. During this brief interval, conditions are again unsteady, and consequently the shape of the lead ring may be deformed or hooked. This can result in the lead end becoming snarled on the reforming tub's mandrel 26.
Attempts have been made at resolving the above-described problems by varying the operating speed of the laying head to suit changing operating conditions. However, at mill delivery speeds of 100 m/sec. and higher, laying head inertia is difficult if not impossible to overcome during the narrow time frame within which front and tail ends are travelling in an unsteady state.